Prerequisites

Before you start working with XSLT, make sure that XPath View+XSLT Support plugin is enabled.
The plugin is bundled with PhpStorm and activated by default. If it is not, enable the plugin as described in the section Enabling and Disabling Plugins.

XSLT support is available in all XML files that declare the XSLT-Namespace
http://www.w3.org/1999/XSL/Transform on their root element.

Overview

The plugin lets you do the following:

Evaluate expressions against the currently focused document, including support for the
document() function to make cross-document queries

Evaluate an expression against multiple XML
documents in a Find in Path style way by the new Find by XPath action.

The Expression Evaluation dialog allows you to
highlight matching expressions in the current editor or to open the Find
tool window to display a list of matching lines. Editing XPath expressions is enhanced by on-the-fly error-checking
including a set of customizable XPath Inspections and a wide range of code
completion suggestions.

The plugin also offers a way to display a unique XPath expression for a selected element in an editor. This is
available via View | Unique XPath in the Main Menu

XSLT support is not limited to XPath expressions, it also supports a wide range of XSLT constructs, like checking
the existence of templates that are called via xsl:call-template and their parameters, refactoring and
navigation enhancements.
Find out more about this in the XSLT Support section.